The application is a national stage entry of PCT/GB00/03124, filing date Aug. 14, 2000.
The present invention is concerned with a method of monitoring the content of peroxide present in a fluid sample, and apparatus for such a method.
Hydrogen peroxide has been used as an oxidant in industrial applications for many years. Hydrogen peroxide is, for example, a stronger oxidant than chlorine or permanganate and has the advantage of non-polluting decomposition products.
Hydroxyl radicals (OHxe2x80xa2) are highly oxidising species. The most commonly accepted mechanism for hydroxyl radical production is the photolysis of hydrogen peroxide. Photochemical reduction of Fe3+ to Fe2+ (UV/Fentons) in the presence of hydrogen peroxide, increases the generation of OHxe2x80xa2 radicals and may yield a more effective system for oxidative degradation.
A major application of peroxide is in advanced oxidation processes removing recalcitrant organic contaminants, such as herbicides and PCB""s (polychlorinated biphenyls). For example, the purification of water containing organic impurities by peroxide (approx 1%)/UV treatment has been used since the early 1980""s. In addition, peroxide together with UV and O3, has been used at a field scale to treat ground water contaminated with volatile organic compounds.
The partial oxidation of recalcitrant compounds may also be advantageous. It has been shown by Carberry and Benzing (Water Sci. Tech 23, 1991, 367-376) that two chlorinated aromatics showed enhanced biodegradability after pre-oxidation with peroxide at molar ratios between 2:1 and 6:1, with an optimum at 4:1.
The use of hydrogen peroxide as an oxidant has several advantages over other methods of chemical and photochemical water treatments, namely its thermal stability, the ability to store on-site, its solubility in water, and the lack of mass transfer problems of associated gases.
Peroxides are used in the removal of color, especially as a bleaching agent in the textile industry. In addition, peroxides are used in the manufacture of paper, and during waste paper recycling.
Other environmental applications include the oxidation of sulfides for odor control, corrosion control of waste pipes by addition of hydrogen peroxide to waste water, an additional oxygen source for overloaded activated sludge plants and controlling filamentous bulking.
It can be seen from the above that the use of hydrogen peroxide in industry has numerous advantages. However, the concentration of peroxide employed in the industrial processes must be carefully controlled and monitored for its efficient and cost effective usage.
There are many methods of monitoring hydrogen peroxide known in the art. Standard methods of monitoring hydrogen peroxide include titrimetric (typically based on the oxidation of hydrogen peroxide with permanganate, followed by the reduction with acidic potassium iodide), gasometric, electrochemical calorimetric, chemiluminescent and acoustic methods. The results of these monitoring methods can then be used to control the process.
The methods outlined above can be time consuming, sensitive to interference, and have poor lifetime. They may not be so effective for process monitoring and control.
PCT patent specification WO98/30884 to BTG Kxc3xa4lle Inventing AB (BTG) discloses a method and device for measuring the content of chemicals (such as hydrogen peroxide) used in connection with bleaching of cellulose fibres. The method includes adding the enzyme catalase to the sample, which is agitated so as to permit the hydrogen peroxide to decompose and oxygen gas to be generated. The resultant oxygen gas pushes out a certain sample volume for the measurement of the sample; the sample volume is then, directly or indirectly, converted to a value representing the amount of hydrogen peroxide present.
In addition, the above mentioned patent specification suggests that it is not possible to measure directly the volume of oxygen produced.
These and other needs in the art are addressed by a method of determining the hydrogen peroxide content of a fluid, which method includes;
(a) contacting the fluid with a catalyst so as to permit decomposition of the hydrogen peroxide present in the fluid to oxygen and water;
(b) permitting the oxygen liberated to pass to a gas meter; and
(c) measuring the volume of oxygen liberated utilizing the gas meter,
wherein the volume of oxygen liberated provides a measure of the hydrogen peroxide content of the fluid.
The term xe2x80x9cfluidxe2x80x9d is a term generally used in the art for any material which can be pumped. Non-limiting examples of such fluids are solutions, slurries, pulps, gravel, etc.
It is preferred that the gas meter is arranged to measure low and irregular gas flows with a low back pressure. Typically, the gas meter is a low flow gas meter, such as the meter described in xe2x80x9cOn-line low flow high precision gas metering systemsxe2x80x9d, Wat. Res, vol 29, page 977-979 (1995), the disclosure of which is incorporated by reference.
The volume may be measured as an absolute volume, or as a rate of evolution (in other words, the volume evolved in a unit time).
It is preferred that the volume of oxygen liberated in step (b) is measured directly.
It is preferred that the catalyst is an enzyme, such as catalase, which may be either soluble or immobilised. However other suitable catalysts may be used. Preferably, the catalase is Hydrogen peroxide: hydrogen peroxide oxidoreductase, EC 1.11.1.6. The catalase catalyses the decomposition of hydrogen peroxide to water and oxygen gas.
Typically, the amount of catalase present in step (a) is predetermined. It is also preferred that the catalase is present in an amount excess relative to the hydrogen peroxide.
It is preferred that the mixing and catalysed release of oxygen in step (a) is carried out for sufficient time to decompose substantially all of the hydrogen peroxide present in the fluid.
The temperature of the process may be kept at a predetermined temperature, for example, in the range from 20xc2x0 C. to 40xc2x0 C.
Preferably, the oxygen liberated may be measured using a pressure transducer or the like, such as a low-flow gas meter. Thus, the hydrogen peroxide content of the sample at any given time may be determined, for example, in a continuous treatment process.
Preferably, the measurements by the gas meter of the oxygen liberated from the sample are passed to a data acquisition system for processing, where, advantageously, the information may be calibrated to produce an accurate reading of the hydrogen peroxide present in the sample.
Advantageously, when it is required to measure small amounts of hydrogen peroxide, such as about 25-500 mg.1xe2x88x921, the fluid sample is aerated prior to contacting with the catalyst.
The method according to the invention may be used in a wide range of industrial applications, for example, in paper processing, textile processes, steel industry and water treatment.
According to a second aspect of the present invention, there is provided apparatus for determining the hydrogen peroxide content of a fluid, which apparatus includes:
(a) a first receptacle arranged to (i) receive the fluid and a catalyst, and (ii) to permit decomposition of the hydrogen peroxide present to oxygen and water;
(b) a gas meter arranged to measure the oxygen evolved.
It is preferred that the flow of fluid and the catalyst are controlled using at least one suitable peristaltic pump and/or at least one centrifugal pump.
In a first embodiment of the second aspect of the present invention, the apparatus is suitable for use in continuous measurements. In this embodiment, the fluid in the receptacle is maintained at a volume of about 15-60 cm3, preferably about 50 cm3. The volume may be advantageously controlled using a manometer.
The apparatus preferably includes aeration means when it is required to measure small amounts of hydrogen peroxide, such as about 25-500 mg.1xe2x88x921.
In a second embodiment of the second aspect of the present invention, the apparatus is suitable for use in batch measurements. In this embodiment, the fluid in the receptacle is maintained at about 25-2000 cm3, preferably about 1000 cm3.
According to a third embodiment of the present invention, there is provided a gas flow meter for use in the measurement of oxygen which is liberated as a result of the decomposition of hydrogen peroxide, the decomposition being a result of contacting the hydrogen peroxide with a catalyst.
It is preferred that the gas meter comprises a three way solenoid valve, a sensitive differential pressure transducer and a ballast chamber.
The pressure transducer is preferably based on a steel diaphragm which advantageously is not significantly affected by temperature changes and is resistant to corrosion by water-saturated gases. Preferably, the solenoid valve comprises a dry paddle solenoid valve, typically made from a material which is resistant to corrosive gases. Such a valve is advantageously corrosion resistant.
The output from the differential pressure transducer may, in some embodiments, be smoothed, typically by an RC circuit or other conventional methods.
It is therefore an aim of the present invention to provide a rapid method to monitor hydrogen peroxide over a wide range of concentrations.
It is a further aim of the present invention to provide a method for on-line monitoring of hydrogen peroxide which utilizes direct measurement of the oxygen gas produced in the decomposition of hydrogen peroxide.
It is yet a further aim of the present invention to provide a monitor which is capable of monitoring hydrogen peroxide in xe2x80x9cdirtyxe2x80x9d and xe2x80x9chigh solid contentxe2x80x9d environments such as industrial process streams and effluents without fouling or interference which is one of the major drawbacks of currently available instruments.